1. Field of the Invention
This invention relates to a marine propulsion unit for a watercraft, and more particularly to a hydraulic tilt and trim adjustment system for a marine propulsion unit.
2. Description of Related Art
The optimal trim angle of an outboard motor varies with a watercraft's running condition. For instance, the bow of the watercraft should press against the water when accelerating from rest or from a slow speed. To achieve this condition, the angle of the propeller shaft is disposed at a negative angle relative to the horizontal (i.e., at a negative trim angle). A thrust vector produced by the propeller in this position is thus out of the water. When running at high speed, the propeller is raised or trimmed to position the propeller shaft at a positive trim angle relative to the horizontal within the range of about 0.degree. to 15.degree.. The outboard motor also must be raised beyond the normal trim range in order to operate in shallow water and for storage in a fill tilt-up position
A hydraulic tilt and trim adjustment system often supports an outboard motor on a watercraft, and adjusts the trim and tilt position of the outboard motor. The tilt and trim adjustment system usually includes at least one hydraulic actuator which operates between a clamping bracket and a swivel bracket. The clamping bracket is attached to the watercraft and the swivel bracket supports the outboard motor. A pivot pin connects the swivel and clamping brackets together. The actuator causes the swivel bracket to pivot about the axis of the pivot pin relative to the stationary clamping bracket to raise or lower the outboard drive.
Tilt and trim adjustment systems also usually employ a hydraulic motor that affects the trim and tilt operations of the outboard drive. For this purpose, prior hydraulic motors have included a reversible electric motor that selectively drives a reversible fluid pump. The pump pressurizes or depressurizes the hydraulic actuator for raising or lowering the outboard drive.
In particular, the fluid pump supplies pressurized fluid to various ports of the actuator's closed cylinder, on either side of a piston which slides within the cylinder. The piston forms separate chambers within the cylinder. A conventional seal, such as one or more O-rings, operates between the piston and cylinder bore to prevent flow from between the chambers. The piston moves within the cylinder by pressurizing the chamber on one side of the piston and depressuring the other chamber on the opposite side.
An actuator arm is attached to the piston and to the swivel bracket. The other end of the cylinder is attached to the clamping bracket. By pressurizing and depressurizing the chambers within the actuator, the piston and thus the outboard motor can be moved.
The pressures in the cylinder chambers vary greatly depending on whether the propulsion unit is operating in a trim range or in a tilt range. In a tilt range, usually associated with tilting the propulsion unit out of the water, the pump generates a relatively low pressure in the chambers because the only load on the cylinder is the weight of the propulsion unit.
The pump conversely must generate far greater pressure to trim-up the motor because of the load placed on the unit by the propulsion unit. The increase in load results from the thrust of the propulsion unit. That is, a portion of the thrust produced by the propulsion unit acts downward and against the tilt and trim mechanism when trimming up. Higher pressures therefore are required in the cylinder to trim up the motor when running at high speeds (e.g., planning speeds).
Prior tilt and trim mechanism have included a relief valve to prevent too much pressure from building within the cylinder. Not only can such pressure damage internal seals, fittings and components of the tilt and trim mechanism, it also can cause the outboard motor to "pop-up" quickly.
Undesirable motor pop-up occurs because the thrust of the propulsion system sudden decreases as the motor is swung through the tilt range. Within the tilt range, the large pressure built-up within the cylinder rapidly pushes the piston upward and causes the outboard motor to pop-up quickly. By properly setting the relief valve, this phenomenon tends not to occur.
Prior tilt and trim adjustment mechanisms have selected the relief valve to open at a pressure just above that associated with running the outboard motor at full throttle and under a fully trimmed-up condition. For larger outboard motors (e.g., 200 hp) this valve design works well. At medium and low speeds, the outboard motor can be trimmed up into the tilt range to accommodate running in shallow waters. (The pop-up phenomenon tends not occur at these speeds because the pressure within the cylinder required to trim up the motor is much less than that when running at high speeds.) The relief valve remains closed under these running conditions.